The material requirements for gas turbine engines are continually being increased. Components formed from powder metal gamma prime (.gamma.') precipitation strengthened nickel-base superalloys can provide a good balance of creep, tensile and fatigue crack growth properties to meet these performance requirements. Typically, a powder metal component is produced by some form of consolidation, such as extrusion consolidation. The resulting billet is then isothermally forged at temperatures slightly below the .gamma.' solvus temperature of the alloy to approach superplastic forming conditions, which allows the filling of the die cavity through the accumulation of high geometric strains without the accumulation of significant metallurgical strains. These processing steps are designed to retain a fine grain size within the material, avoid fracture during forging, and maintain relatively low forging loads. In order to improve the fatigue crack growth resistance and mechanical properties of these materials at elevated temperatures, these alloys are then heat treated above their .gamma.' solvus temperature (generally referred to as supersolvus heat treatment), to cause significant, uniform coarsening of the grains.
However, during conventional manufacturing procedures involving hot forging operations, a wide range of local strains and strain rates may be introduced into the material which cause non-uniform critical grain growth during post forging supersolvus heat treatment. Critical grain growth as used herein refers to localized abnormal excessive grain growth in an alloy which results in the formation of grains whose diameters exceed a desired grain size range for an article formed from the alloy. Accordingly, the term "uniform" with respect to grain size and growth refers to the substantial absence of critical grain growth. Desired ranges for gas turbine engine components often entail grain sizes of ASTM 9 and coarser, but are generally limited to a range of several ASTM units in order to be considered uniform. (Reference throughout to ASTM grain sizes is in accordance with the standard scale established by the American Society for Testing and Materials.)
The presence of grains within a component which significantly exceed the desired grain size range are highly undesirable, in that the presence of such grains can significantly reduce the low cycle fatigue resistance of the article and can have a negative impact on other mechanical properties of the article, such as tensile and fatigue strength. As an example, if the desired grain size range for a nickel-base superalloy article is ASTM 7 to ASTM 8, random grain growth which produces grains coarser than about ASTM 4 will often be undesirable.
The propensity for critical grain growth increases if more conventional cast and wrought processing or spraycast forming techniques are used to form such components. As such, critical components are generally formed from powder metallurgy particles which have been extrusion consolidated. However, even these components are susceptible to critical grain growth during supersolvus heat treatment, particularly if the component has an extremely complex shape or is formed by friction welding two or more components together, as in the case of some turbine disks.
U.S. Pat. No. 4,957,567 to Krueger et al., assigned to the same assignee of the present patent application, eliminates critical grain growth in fine grain nickel-base superalloy components by controlling the localized strain rates experienced during the hot forging operations. Krueger et al. teach that, generally, local strain rates must remain below a critical value, .sub.c, in order to avoid detrimental critical grain growth during subsequent supersolvus heat treatment. Strain rate is defined as the instantaneous rate of change of geometric strain with time.
However, critical grain growth can occur if the processing parameters of the alloy during forging and heat treatment are not properly controlled. As such, the process window for many components is relatively narrow, resulting in increased costs due to scrappage. Accordingly, it would be desirable to identify processing parameters which further enable the production of nickel-base superalloy articles having a uniform grain microstructure, so as to enhance the processibility of nickel-base superalloys in order to achieve desirable microstructures.